The toxicity of showdomycin toward both L1210 cells and bone marrow progenitor cells in vitro can be blocked by the nucleoside transport inhibitor 6-((2-hydroxy-5-nitrobenzyl)-thio)purine-9-B-D-ribofuranoside. However, the cytotoxicity of the chemotherapeutic agent, 5-azacytidine cannot be reduced in either cell line.